Geology topics

Robert J. McLaughlin

Publications and source records attributed to Robert J. McLaughlin.

At least 19 recordsLinked to original sources

USGS2024

The geometry of fault reactivation and uplift along the central part of the Maacama fault zone, northern California Coast Ranges (USA)

Fault reactivation of bedrock structures in active fault zones influences stress state and earthquake rupture phenomena through the introduction of weak slip surfaces that impact fault zone geometry and width. Yet, geometric relationships between modern faults and older reactivated faults are difficult to quantify in rocks that have experienced multiple deformation episodes. We used new geologic mapping, geomorphic tools, and structural modeling to quantify rock uplift and subsurface fault geometry of the central part of the Maacama Fault Zone near Ukiah, California, USA, and the surrounding area. Results suggest that the northern Mayacamas Mountains are in a tectonically driven disequilibrium, with differential rock uplift focused on the western side of the range. Steeply east-dipping fault surfaces and splays characterize the geometry of the Maacama Fault Zone. We mapped two newly identified faults to the east of the main Maacama Fault, the Cow Mountain–Mill Creek Fault, and Willow Creek Fault, which align with a moderately east-dipping cluster of microseismicity between 4–10 km depth beneath the Mayacamas Mountains. Static stress modeling on the Maacama Fault Zone and newly identified faults to the east quantify slip tendency values of 0.5–0.4, which suggests that the faults are moderately to poorly suited for slip in the modern stress field and may be weak. We infer that modern uplift is driven by oblique reverse, up-to-the-east, dip-slip motion on the reactivated Cenozoic Cow Mountain–Mill Creek and Willow Creek Faults as material is advected through a restraining bend on the Maacama Fault. This study shows that reactivated bedrock faults increase the fault zone width and introduce fault surfaces that contribute a component of vertical deformation and uplift in major strike-slip fault zones. Deformation is accommodated on an interconnected network of new and reactivated faults that delineate a complex seismic hazard.

California↗

USGS2023

Integrated geologic and geophysical modeling across the Bartlett Springs fault zone, northern California (USA): Implications for fault creep and regional structure

The rate and location at depth of fault creep are important, but difficult to characterize, parameters needed to assess seismic hazard. Here we take advantage of the magnetic properties of serpentinite, a rock type commonly associated with fault creep, to model its depth extent along the Bartlett Springs fault zone, an important part of the San Andreas fault system north of the San Francisco Bay, California (western United States). We model aeromagnetic and gravity anomalies using geologic constraints along 14 cross sections over a distance of 120 km along the fault zone. Our results predict that the fault zone has more serpentinite at depth than inferred by geologic relationships at the surface. Existing geodetic models are inconsistent and predict different patterns of creep along the fault. Our results favor models with more extensive creep at depth. The source of the serpentinite appears to be ophiolite thrust westward and beneath the Franciscan Complex, an interpretation supported by the presence of antigorite, a high-temperature serpent ine mineral stable at depth, in fault gouge near Lake Pillsbury.

California↗

USGS2021

Comment on ‘New insights on Franciscan Complex geology, architecture, depositional age, and provenance for the western Mt. Tamalpais area, Marin County, California’ by Bero et al. (2020)

Serious errors and inconsistencies in the article undermine many of its interpretations to the point that principal conclusions are not valid. Much dependence is placed on the maximum depositional age (Dmax) of sandstone units based on zircon analysis of 10 samples, but calculation of those Dmax values is flawed, and their use confuses maximum with actual depositional ages and makes age distinctions finer than the resolution of the data. Conclusions that are compromised include the concept of a westward/downward-younging tectonostratigraphic stack of accretionary units, comparison of mapped sandstones with other California Coast Ranges sandstones, comparison of timing of events recorded in the map area with regional events, identification of mélange only in a narrow band in the study area, and detrital provenance of some of the sandstone units.

California↗

USGS2018

Serpentinite‐rich gouge in a creeping segment of the Bartlett Springs Fault, northern California: Comparison with SAFOD and implications for seismic hazard

An exposure of a creeping segment of the Bartlett Springs Fault (BSF), part of the San Andreas Fault system in northern California, is a ~1.5‐m‐wide zone of serpentinite‐bearing fault gouge cutting through Late Pleistocene fluvial deposits. The fault gouge consists of porphyroclasts of antigorite serpentinite, talc, chlorite, and tremolite‐actinolite, along with some Franciscan metamorphic rocks, in a matrix of the same materials. The Mg‐mineral assemblage is stable at temperatures above 250–300 °C. The BSF gouge is interpreted to have been tectonically incorporated into the fault from depths near the base of the seismogenic zone and to have risen buoyantly to the surface where it is now undergoing right‐lateral displacement. The ultramafic‐rich composition, frictional properties, and inferred mode of emplacement of the BSF serpentinitic gouge correspond to those of the creeping traces of the San Andreas Fault identified in the SAFOD (San Andreas Fault Observatory at Depth) drill hole. This suggests a common origin for creep at both locations. A tectonic model for the source of the ultramafic‐rich materials in the BSF is proposed that potentially could explain the distribution of creep throughout the northernmost San Andreas Fault system.

California↗

USGS2018

Framework geologic map and structure sections along the Bartlett Springs fault zone and adjacent area from Round Valley to Wilbur Springs, northern Coast Ranges, California

The geologic map and accompanying report describes the extent, complexity, architecture, and evolution of the Bartlett Springs Fault Zone between Clear Lake and Round Valley, California. This fault zone is the eastern-most known active member of the San Andreas transform margin in northern California. It is of particular interest for its apparent long-lived history as a Miocene and older subduction-margin fault that, more recently, was reactivated as an active, creeping member of the San Andreas Fault system. The northern part of the Bartlett Springs Fault Zone is apparently still influenced by subduction of the Gorda Plate beneath North America, but it also accommodates strike-slip displacement associated with interaction of the Pacific Plate with North America. South of the map area, the Bartlett Springs Fault Zone steps into and merges with active faults of the eastern San Francisco Bay region; to the north of the map area and Round Valley, the fault zone steps into several other fault zones that connect with offshore thrust faults of the Cascadia subduction margin. Adequate understanding of the geologic framework of this fault zone and its relation to crustal structure of the adjacent region is important for purposes of planning and upgrading hydro-electric and other infrastructure in northern California that is directly or indirectly impacted by active faulting.

California↗

USGS2015

Detrital zircon U-Pb reconnaissance of the Franciscan subduction complex in northwestern California

In northwestern California, the Franciscan subduction complex has been subdivided into seven major tectonostratigraphic units. We report U-Pb ages of ≈2400 detrital zircon grains from 26 sandstone samples from 5 of these units. Here, we tabulate each unit's interpreted predominant sediment source areas and depositional age range, ordered from the oldest to the youngest unit. (1) Yolla Bolly terrane: nearby Sierra Nevada batholith (SNB); ca. 118 to 98 Ma. Rare fossils had indicated that this unit was mostly 151-137 Ma, but it is mostly much younger. (2) Central Belt: SND; ca. 103 too 53 Ma (but poorly constrained), again mostly younger than previously thought. (3) Yager terrane: distant Idaho batholith (IB); ca. 52 to 50 Ma. Much of the Yager's detritus was shed during major core complex extension and erosion in Idaho that started 53 Ma. An eocene Princeton River-Princeton submarine canyon system transported this detritus to the Great Valley forearc basin and thence to the Franciscan trench. (4) Coastal terrane: mostly IB, ±SNB, ±nearby Cascade arc, ±Nevada Cenozoic ignimbrite belt; 52 to <32 Ma. (5) King Range terrane: dominated by IB and SNB zircons; parts 16-14 Ma based on microfossils. Overall, some Franciscan units are younger than previously thought, making them more compatible with models for the growth of subduction complexes by positive accretion. From ca. 118 to 70 Ma, Franciscan sediments were sourced mainly from the nearby Sierra Nevada region and were isolated from southwestern US and Mexican sources. From 53 to 49 Ma, the Franciscan was sourced from both Idaho and the Sierra Nevada. By 37-32 Ma, input from Idaho had ceased. The influx from Idaho probably reflects major tectonism in Idaho, Oregon, and Washington, plus development of a through-going Princeton River to California, rather than radical changes in the subduction system at the Franciscan trench itself.

California↗

USGS2015

A summary of the late Cenozoic stratigraphic and tectonic history of the Santa Clara Valley, California

The late Cenozoic stratigraphic and tectonic history of the Santa Clara Valley illustrates the dynamic nature of the North American–Pacific plate boundary and its effect on basin and landscape development. Prior to early Miocene time, the area that became Santa Clara Valley consisted of eroding Franciscan complex basement structurally interleaved in places with Coast Range ophiolite and Mesozoic Great Valley sequence, and locally overlapped by Paleogene strata. During early to middle Miocene time, this landscape was flooded by the sea and was deformed locally into deeper depressions such as the Cupertino Basin in the southwestern part of the valley. Marine deposition during the middle and late Miocene laid down thin deposits in shallow water and thick deeper-water deposits in the Cupertino Basin. During this sedimentation, the San Andreas fault system encroached into the valley, with most offset partitioned onto the San Andreas fault southwest of the valley and the southern Calaveras–Silver Creek–Hayward fault system in the northeastern part of the valley. A 6-km-wide right step between the Hayward and Silver Creek faults formed the 40-km-long Evergreen pull-apart basin along the northeastern margin of the valley, leaving a basement ridge between it and the Cupertino Basin. The Silver Creek fault was largely abandoned ca. 2.5 Ma in favor of a compressional left step between the Calaveras and Hayward fault, although some slip continued to at least mid-Quaternary time. Gravity, seismic, stratigraphic, and interferometric synthetic aperture radar (InSAR) data indicate no other major San Andreas system faults within the central block between the present-day range-front faults bounding the valley and the Silver Creek fault. Sometime between 9 and 4 Ma (9 and 1 Ma for the central block), the area rose above sea level, and a regional surface of erosion was carved into the Mesozoic and Tertiary rocks. Alluvial gravels were deposited on this surface along the margins of the valley beginning ca. 4 Ma, but they may not have prograded onto the central block until ca. 1 Ma, because no older equivalents of the Pliocene–Quaternary Santa Clara gravels have been found there. Thus, either the central block was high enough relative to the surrounding areas that Santa Clara gravels were never deposited on it, or any Santa Clara gravels deposited there were stripped away before ca. 1 Ma. Analysis of alluvium on the central block implies a remarkably uniform, piston-like, subsidence of the valley of ∼0.4 mm/yr since ca. 0.8 Ma, possibly extending north to northern San Francisco Bay. Today, the central block continues to subside, the range-front reverse faults are active, and the major active faults of the San Andreas system are mostly outside the valley.

California↗

USGS2013

Previously unrecognized regional structure of the Coastal Belt of the Franciscan Complex, northern California, revealed by magnetic data

Magnetic anomalies provide surprising structural detail within the previously undivided Coastal Belt, the westernmost, youngest, and least-metamorphosed part of the Franciscan Complex of northern California. Although the Coastal Belt consists almost entirely of arkosic graywacke and shale of mainly Eocene age, new detailed aeromagnetic data show that it is pervasively marked by long, narrow, and regularly spaced anomalies. These anomalies arise from relatively simple tabular bodies composed principally of magnetic basalt or graywacke confi ned mainly to the top couple of kilometers, even though metamorphic grade indicates that these rocks have been more deeply buried, at depths of 5–8 km. If true, this implies surprisingly uniform uplift of these rocks. The basalt (and associated Cretaceous limestone) occurs largely in the northern part of the Coastal Belt; the graywacke is recognized only in the southern Coastal Belt and is magnetic because it contains andesitic grains. The magnetic grains were not derived from the basalt, and thus require a separate source. The anomalies defi ne simple patterns that can be related to folding and faulting within the Coastal Belt. This apparent simplicity belies complex structure mapped at outcrop scale, which can be explained if the relatively simple tabular bodies are internally deformed, fault-bounded slabs. One mechanism that can explain the widespread lateral extent of the thin layers of basalt is peeling up of the uppermost part of the oceanic crust into the accretionary prism, controlled by porosity and permeability contrasts caused by alteration in the upper part of the subducting slab. It is not clear, however, how this mechanism might generate fault-bounded layers containing magnetic graywacke. We propose that structural domains defined by anomaly trend, wavelength, and source reflect imbrication and folding during the accretion process and local plate interactions as the Mendocino triple junction migrated north, a hypothesis that should be tested by more detailed structural studies.

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USGS2012

Evolution of the Rodgers Creek–Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern California

The Rodgers Creek–Maacama fault system in the northern California Coast Ranges (United States) takes up substantial right-lateral motion within the wide transform boundary between the Pacific and North American plates, over a slab window that has opened northward beneath the Coast Ranges. The fault system evolved in several right steps and splays preceded and accompanied by extension, volcanism, and strike-slip basin development. Fault and basin geometries have changed with time, in places with younger basins and faults overprinting older structures. Along-strike and successional changes in fault and basin geometry at the southern end of the fault system probably are adjustments to frequent fault zone reorganizations in response to Mendocino Triple Junction migration and northward transit of a major releasing bend in the northern San Andreas fault. The earliest Rodgers Creek fault zone displacement is interpreted to have occurred ca. 7 Ma along extensional basin-forming faults that splayed northwest from a west-northwest proto-Hayward fault zone, opening a transtensional basin west of Santa Rosa. After ca. 5 Ma, the early transtensional basin was compressed and extensional faults were reactivated as thrusts that uplifted the northeast side of the basin. After ca. 2.78 Ma, the Rodgers Creek fault zone again splayed from the earlier extensional and thrust faults to steeper dipping faults with more north-northwest orientations. In conjunction with the changes in orientation and slip mode, the Rodgers Creek fault zone dextral slip rate increased from ∼2–4 mm/yr 7–3 Ma, to 5–8 mm/yr after 3 Ma. The Maacama fault zone is shown from several data sets to have initiated ca. 3.2 Ma and has slipped right-laterally at ∼5–8 mm/yr since its initiation. The initial Maacama fault zone splayed northeastward from the south end of the Rodgers Creek fault zone, accompanied by the opening of several strike-slip basins, some of which were later uplifted and compressed during late-stage fault zone reorganization. The Santa Rosa pull-apart basin formed ca. 1 Ma, during the reorganization of the right stepover geometry of the Rodgers Creek–Maacama fault system, when the maturely evolved overlapping geometry of the northern Rodgers Creek and Maacama fault zones was overprinted by a less evolved, non-overlapping stepover geometry. The Rodgers Creek–Maacama fault system has contributed at least 44–53 km of right-lateral displacement to the East Bay fault system south of San Pablo Bay since 7 Ma, at a minimum rate of 6.1–7.8 mm/yr.

Geosphere↗

USGS2012

Mineral parageneses, regional architecture, and tectonic evolution of Franciscan metagraywackes, Cape Mendocino-Garberville-Covelo 30' x 60' quadrangles, northwest California

The Franciscan Complex is a classic subduction-zone assemblage. In northwest California, it comprises a stack of west vergent thrust sheets: westernmost Eastern Belt outliers; Central Belt mélange; Coastal Belt Yager terrane; Coastal Belt Coastal terrane; Coastal Belt King Range/False Cape terranes. We collected samples and determined P-T conditions of recrystallization for 88 medium-fine-grained metasandstones to assess their subduction-exhumation histories and assembly of the host allochthons. Feebly recrystallized Yager, Coastal, and King Range strata retain clear detrital features. Scattered neoblastic prehnite occurs in several Coastal terrane metasandstones; traces of possible pumpellyite are present in three Yager metaclastic rocks. Pumpellyite ± lawsonite ± aragonite-bearing Central Belt metasandstones are moderately deformed and reconstituted. Intensely contorted, thoroughly recrystallized Eastern Belt affinity quartzose metagraywackes contain lawsonite + jadeitic pyroxene ± aragonite ± glaucophane. We microprobed neoblastic phases in 23 rocks, documenting mineral parageneses that constrain the tectonic accretion and metamorphic P-T evolution of these sheets. Quasi-stable mineral assemblages typify Eastern Belt metasandstones, but mm-sized domains in the Central and Coastal belt rocks failed to achieve chemical equilibrium. Eastern Belt slabs rose from subduction depths approaching 25–30 km, whereas structurally lower Central Belt mélanges returned from &sim;15–18 km. Coastal Belt assemblages suggest burial depths less than 5–8 km. Eastern and Central belt allochthons sequentially decoupled from the downgoing oceanic lithosphere and ascended into the accretionary margin; K-feldspar-rich Coastal Belt rocks were stranded along the continental edge without undergoing appreciable subduction, probably during Paleogene unroofing of the older, deeply subducted units of the Franciscan Complex in east-vergent crustal wedges.

California↗

USGS2011

Geology, geochronology, and paleogeography of the southern Sonoma volcanic field and adjacent areas, northern San Francisco Bay region, California

Recent geologic mapping in the northern San Francisco Bay region (California, USA) supported by radiometric dating and tephrochronologic correlations, provides insights into the framework geology, stratigraphy, tectonic evolution, and geologic history of this part of the San Andreas transform plate boundary. There are 25 new and existing radiometric dates that define three temporally distinct volcanic packages along the north margin of San Pablo Bay, i.e., the Burdell Mountain Volcanics (11.1 Ma), the Tolay Volcanics (ca. 10–8 Ma), and the Sonoma Volcanics (ca. 8–2.5 Ma). The Burdell Mountain and the Tolay Volcanics are allochthonous, having been displaced from the Quien Sabe Volcanics and the Berkeley Hills Volcanics, respectively. Two samples from a core of the Tolay Volcanics taken from the Murphy #1 well in the Petaluma oilfield yielded ages of 8.99 ± 0.06 and 9.13 ± 0.06 Ma, demonstrating that volcanic rocks exposed along Tolay Creek near Sears Point previously thought to be a separate unit, the Donnell Ranch volcanics, are part of the Tolay Volcanics. Other new dates reported herein show that volcanic rocks in the Meacham Hill area and extending southwest to the Burdell Mountain fault are also part of the Tolay Volcanics. In the Sonoma volcanic field, strongly bimodal volcanic sequences are intercalated with sediments. In the Mayacmas Mountains a belt of eruptive centers youngs to the north. The youngest of these volcanic centers at Sugarloaf Ridge, which lithologically, chemically, and temporally matches the Napa Valley eruptive center, was apparently displaced 30 km to the northwest by movement along the Carneros and West Napa faults. The older parts of the Sonoma Volcanics have been displaced at least 28 km along the Rodgers Creek fault since ca. 7 Ma. The Petaluma Formation also youngs to the north along the Rodgers Creek–Hayward fault and the Bennett Valley fault. The Petaluma basin formed as part of the Contra Costa basin in the Late Miocene and was displaced to its present location along the Rodgers Creek–Hayward and older faults. The Tolay fault, previously thought to be a major dextral fault, is part of a fold-and-thrust belt that does not exhibit lateral displacement.

California↗

USGS2011

Significance of zircon U-Pb ages from the Pescadero felsite, west-central California coast ranges

Weathered felsite is associated with the late Campanian–Maastrichtian Pigeon Point Formation near Pescadero, California. Poorly exposed, its age and correlation are uncertain. Is it part of the Pigeon Point section west of the San Gregorio–Hosgri fault? Does it rest on Nacimiento block basement? Is it dextrally offset from the Oligocene Cambria Felsite, ∼185 km to the southeast? Why is a calc-alkaline hypabyssal igneous rock intrusive into the outboard accretionary prism? To address these questions, we analyzed 43 oscillatory-zoned zircon crystals from three incipiently recrystallized pumpellyite ± prehnite ± laumontite-bearing Pescadero felsite samples by sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) techniques. Thirty-three zircons gave late Mesozoic U-Pb ages, with single-grain values ranging from 81 to 167 Ma; ten have pre-Mesozoic, chiefly Proterozoic ages. A group of the four youngest Pescadero zircons yielded an apparent maximum igneous age of ca. 86–90 Ma. Reflecting broad age scatter and presence of partly digested sandstone inclusions, we interpret the rest of the zircons (perhaps all) as xenocrysts. Twenty-three zircons were separated and analyzed from two samples of the similar Cambria Felsite, yielding a unimodal 27 Ma U-Pb age. Clearly, the origin of the Upper Oligocene Cambria Felsite is different from that of the Upper Cretaceous Pescadero felsite; these rocks are not correlated, and do not constrain displacement along the San Gregorio–Hosgri fault. Peak ages differ slightly, but relative probability curves for Mesozoic and pre-Mesozoic Pescadero zircons compare well, for example, with abundant U-Pb age data for detrital zircons from Franciscan metaclastic strata ∼100 km to the east in the Diablo Range–San Francisco Bay area, San Joaquin Great Valley Group turbidites, Upper Cretaceous Nacimiento block Franciscan strata, and Upper Cretaceous forearc units of the Transverse Ranges. Based on zircon U-Pb ages, geologic and petrographic relations, the Pescadero felsite and a capping, sheared metaconglomerate underlie the Pigeon Point Formation. We infer that the magma formed by anatexis of Franciscan or Great Valley clastic sedimentary rocks originating from a parental Mesozoic Sierran-Mojave-Salinian calc-alkaline arc. The felsite erupted during Late Cretaceous time, was metamorphosed to pumpellyite-prehnite grade within the subduction zone, and then was rapidly exhumed, weakly zeolitized, and exposed before Pigeon Point forearc deposition. Pescadero volcanism apparently reflects a previously unrecognized ca. 86–90 Ma felsic igneous event in the accretionary margin.

California↗

USGS2011

Age, composition, and areal distribution of the Pliocene Lawlor Tuff, and three younger Pliocene tuffs, California and Nevada

The Lawlor Tuff is a widespread dacitic tephra layer produced by Plinian eruptions and ash flows derived from the Sonoma Volcanics, a volcanic area north of San Francisco Bay in the central Coast Ranges of California, USA. The younger, chemically similar Huichica tuff, the tuff of Napa, and the tuff of Monticello Road sequentially overlie the Lawlor Tuff, and were erupted from the same volcanic field. We obtain new laser-fusion and incremental-heating 40 Ar/ 39 Ar isochron and plateau ages of 4.834 ± 0.011, 4.76 ± 0.03, ≤4.70 ± 0.03, and 4.50 ± 0.02 Ma (1 sigma), respectively, for these layers. The ages are concordant with their stratigraphic positions and are significantly older than those determined previously by the K-Ar method on the same tuffs in previous studies. Based on offsets of the ash-flow phase of the Lawlor Tuff by strands of the eastern San Andreas fault system within the northeastern San Francisco Bay area, total offset east of the Rodgers Creek–Healdsburg fault is estimated to be in the range of 36 to 56 km, with corresponding displacement rates between 8.4 and 11.6 mm/yr over the past ∼4.83 Ma. We identify these tuffs by their chemical, petrographic, and magnetic characteristics over a large area in California and western Nevada, and at a number of new localities. They are thus unique chronostratigraphic markers that allow correlation of marine and terrestrial sedimentary and volcanic strata of early Pliocene age for their region of fallout. The tuff of Monticello Road is identified only near its eruptive source.

California, Nevada↗

USGS2010

Neogene contraction between the San Andreas fault and the Santa Clara Valley, San Francisco Bay region, California

In the southern San Francisco Bay region of California, oblique dextral reverse faults that verge northeastward from the San Andreas fault experienced triggered slip during the 1989 M7.1 Loma Prieta earthquake. The role of these range-front thrusts in the evolution of the San Andreas fault system and the future seismic hazard that they may pose to the urban Santa Clara Valley are poorly understood. Based on recent geologic mapping and geophysical investigations, we propose that the range-front thrust system evolved in conjunction with development of the San Andreas fault system. In the early Miocene, the region was dominated by a system of northwestwardly propagating, basin-bounding, transtensional faults. Beginning as early as middle Miocene time, however, the transtensional faulting was superseded by transpressional NE-stepping thrust and reverse faults of the range-front thrust system. Age constraints on the thrust faults indicate that the locus of contraction has focused on the Monte Vista, Shannon, and Berrocal faults since about 4.8 Ma. Fault slip and fold reconstructions suggest that crustal shortening between the San Andreas fault and the Santa Clara Valley within this time frame is ∼21%, amounting to as much as 3.2 km at a rate of 0.6 mm/yr. Rates probably have not remained constant; average rates appear to have been much lower in the past few 100 ka. The distribution of coseismic surface contraction during the Loma Prieta earthquake, active seismicity, late Pleistocene to Holocene fluvial terrace warping, and geodetic data further suggest that the active range-front thrust system includes blind thrusts. Critical unresolved issues include information on the near-surface locations of buried thrusts, the timing of recent thrust earthquake events, and their recurrence in relation to earthquakes on the San Andreas fault.

California↗

USGS2010

Geophysical framework of the northern San Francisco Bay region, California

We use geophysical data to examine the structural framework of the northern San Francisco Bay region, an area that hosts the northward continuation of the East Bay fault system. Although this fault system has accommodated ∼175 km of right-lateral offset since 12 Ma, how this offset is partitioned north of the bay is controversial and important for understanding where and how strain is accommodated along this stretch of the broader San Andreas transform margin. Using gravity and magnetic data, we map these faults, many of which influenced basin formation and volcanism. Continuity of magnetic anomalies in certain areas, such as Napa and Sonoma Valleys, the region north of Napa Valley, and the region south of the Santa Rosa Plain, preclude significant (>10 km) offset. Much of the slip is partitioned around Sonoma and Napa Valleys and onto the Carneros, Rodgers Creek, and Green Valley faults. The absence of correlative magnetic anomalies across the Hayward–Rodgers Creek–Maacama fault system suggests that this system reactivated older basement structures, which appear to influence seismicity patterns in the region.

California↗

USGS2010

Three-dimensional geologic modeling of the Santa Rosa Plain, California

New three-dimensional (3D) lithologic and stratigraphic models of the Santa Rosa Plain (California, USA) delineate the thickness, extent, and distribution of subsurface geologic units and allow integration of diverse data sets to produce a lithologic, stratigraphic, and structural architecture for the region. This framework can be used to predict pathways of groundwater flow beneath the Santa Rosa Plain and potential areas of enhanced or focused seismic shaking. Lithologic descriptions from 2683 wells were simplified to 19 internally consistent lithologic classes. These distinctive lithologic classes were used to construct a 3D model of lithologic variations within the basin by extrapolating data away from drill holes using a nearest-neighbor approach. Subsurface stratigraphy was defined through the identification of distinctive lithologic packages tied, where possible, to high-quality well control and to surface exposures. The 3D stratigraphic model consists of three bounding components: fault surfaces, stratigraphic surfaces, and a surface representing the top of pre-Cenozoic basement, derived from inversion of regional gravity data. The 3D lithologic model displays a west to east transition from dominantly marine sands to heterogeneous continental sediments. In contrast to previous stratigraphic studies, the new models emphasize the prevalence of the clay-rich Petaluma Formation and its heterogeneous nature. Isopach maps of the Glen Ellen Formation and the 3D stratigraphic model show the influence of the Trenton Ridge, a concealed basement ridge that bisects the plain, on sedimentation; the thickest deposits of the Glen Ellen Formation are confined to north of the Trenton Ridge.

California↗

USGS2010

Geologic map of the Bartlett Springs Fault Zone in the vicinity of Lake Pillsbury and adjacent areas of Mendocino, Lake, and Glenn Counties, California

The Lake Pillsbury area lies in the eastern part of the northern California Coast Ranges, along the east side of the transform boundary between the Pacific and North American plates (fig. 1). The Bartlett Springs Fault Zone is a northwest-trending zone of faulting associated with this eastern part of the transform boundary. It is presently active, based on surface creep (Svarc and others, 2008), geomorphic expression, offset of Holocene units (Lienkaemper and Brown, 2009), and microseismicity (Bolt and Oakeshott, 1982; Dehlinger and Bolt, 1984; DePolo and Ohlin, 1984). Faults associated with the Bartlett Springs Fault Zone at Lake Pillsbury are steeply dipping and offset older low to steeply dipping faults separating folded and imbricated Mesozoic terranes of the Franciscan Complex and interleaved rocks of the Coast Range Ophiolite and Great Valley Sequence. Parts of this area were mapped in the late 1970s and 1980s by several investigators who were focused on structural relations in the Franciscan Complex (Lehman, 1978; Jordan, 1975; Layman, 1977; Etter, 1979). In the 1980s the U.S. Geological Survey (USGS) mapped a large part of the area as part of a mineral resource appraisal of two U.S. Forest Service Roadless areas. For evaluating mineral resource potential, the USGS mapping was published at a scale of 1:62,500 as a generalized geologic summary map without a topographic base (Ohlin and others, 1983; Ohlin and Spear, 1984). The previously unpublished mapping with topographic base is presented here at a scale of 1:30,000, compiled with other mapping in the vicinity of Lake Pillsbury. The mapping provides a geologic framework for ongoing investigations to evaluate potential earthquake hazards and structure of the Bartlett Springs Fault Zone. This geologic map includes part of Mendocino National Forest (the Elk Creek Roadless Area) in Mendocino, Glenn, and Lake Counties and is traversed by several U.S. Forest Service Routes, including M1 and M6 (fig. 2). The study area is characterized by northwest-trending ridges separated by steep-sided valleys. Elevations in this part of the Coast Ranges vary from 1,500 ft (457 m) to 6,600 ft (2,012 m), commonly with gradients of 1,000 ft per mile (90 m per km). The steep slopes are covered by brush, grass, oak, and conifer forests. Access to most of the area is by county roads and Forest Service Route M6 from Potter Valley to Lake Pillsbury and by county road and Forest Service Route M6 and M1 from Upper Lake and State Highway 20. From the north, State Highway 261 provides access from Covelo. Forest Service Route M1 trends roughly north from its intersection with Route M6 south of Hull Mountain and through the Elk Creek and Black Butte Roadless areas to State Highway 261. Side roads used for logging and jeep trails provide additional access in parts of the area.

Open-File Report↗

USGS2006

Biostratigraphic and lithologic correlations of two Sonoma County Water Agency pilot wells with the type Wilson Grove Formation, Sonoma County, central California

Small mollusk faunas characteristic of the uppermost part of the Wilson Grove Formation at Wilson Grove and along River Road at Trenton (Pliocene) were encountered in Sonoma County Water Agency pilot wells at Occidental Road well field between 320-500 ft (98-152 m), depth, and in the Sebastopol Road pilot well field between 560-570 ft (171-174 m), depth. These mollusks support correlations between the two wells made on lithologic grounds. A benthic foraminifer was recovered from between 380-390 ft (116-119 m), depth, in the Sonoma County Water Agency Occidental Road pilot well. Though an isolated specimen, the presence of this well-preserved foraminifer supports the environmental interpretation of less than 100 m on the continental shelf indicated by the molluscan assemblages at this site. For this marine stratigraphic interval of the Wilson Grove Formation, we suggest a relatively narrow age range of 5.3 (Miocene-Pliocene boundary) to ~ 4.5 Ma based on the stratigraphic relations of correlative marine strata around the upland margin of Santa Rosa plain and correlative strata in the Santa Cruz area, although an age between 5.3 and ~ 2.8 Ma cannot be discounted.

Open-File Report↗